1. Field of the Invention
The invention relates to holographic storage, in particular holographic storage processes such as phase correlation multiplexing.
2. Discussion of the Related Art
Holographic memory systems involve the three-dimensional storage of holographic representations (i.e., holograms) of data elements as a pattern of varying refractive index and/or absorption imprinted into the volume of a storage medium. Holographic memory systems are characterized by their high density storage potential and the potential speed with which the stored data is randomly accessed and transferred.
In general, holographic storage memory systems operate by combining a data-encoded object beam with a reference beam to create an interference pattern in a photosensitive storage medium. (See, e.g., D. Psaltis et al., "Holographic Memories," Scientific American, November 1995, the disclosure of which is hereby incorporated by reference.) The interference pattern induces material alterations in the medium that create a hologram. The formation of the hologram in the storage medium is a function of the relative amplitudes and polarization states of, and phase differences between, the object beam and the reference beam. It is also highly dependent on the incident beams' wavelengths and the angles at which the object beam and the reference beam are projected into the storage medium.
Holographically stored data is reconstructed by projecting into the medium a reference beam at the same angle, wavelength, phase, and position as the reference beam used to produce the hologram. The hologram and the reference beam interact to reconstruct the stored object beam. The reconstructed object beam then is detected, e.g., using a photodetector array, and the recovered data is post-processed for delivery to output devices.
Typically, the dynamic range of the holographic storage medium is larger than what is needed to store a single hologram with an acceptable signal-to-noise ratio. Therefore, it is often desirable to multiplex a number of holograms at one location to attain greater storage density. One technique for multiplexing is phase correlation multiplexing, as discussed in co-assigned U.S. Pat. No. 5,719,691, the disclosure of which is hereby incorporated by reference. In phase correlation multiplexing, correlation sensitivity and Bragg selectivity are used to differentiate overlapping holograms within a medium. Correlation selectivity relies on the differences in amplitude, phase, and angle content of the reference beam generated by the relative shift of the storage medium with respect to the reference beam. See also co-assigned U.S. Pat. No. 5,703,705, relating to tilt multiplex holography.
While holographic processes such as phase correlation multiplexing provides useful results, variations and improvements in such processes are actively being sought to provide increased storage density and more accurate readout of stored holograms.